In recent years, active research and development are being conducted on lithium cells, especially rechargeable lithium secondary cells, as a new type of secondary cells with a high voltage and a high energy density. In the early stage of its initial research, great expectation was placed as a high energy-density cell on lithium secondary cells that employed metallic lithium in the negative electrode. However, when metallic lithium is used in the negative electrode, dendritic lithium (dendrite) produced during charge grows with charge and discharge of the cell and eventually causes problems such as internal short-circuit of the cell or even abnormal temperature rise of the cell. In an effort to help solve these problems, trials have been made to use alloys with lithium of low-melting metals such as aluminum, lead, indium, bismuth, cadmium, etc., in the negative electrode rather than using metallic lithium alone. However, as the charge and discharge proceed, the alloys become fine particles and penetrate through the separator, eventually causing internal short-circuit. Lithium secondary cells were thus difficult to commercialize.
Consequently, lithium secondary cells that employ carbon in the negative electrode and lithiated transition metal compound in the positive are becoming the mainstream. In these cell systems, as the charge and discharge are performed by the intercalation and de-intercalation of lithium ions in the carbon of the negative electrode, dendrites will not grow during charge, and cells with a good cycle characteristic and superior safety have become available.
In recent years, with the rapid advance in portable or cordless electronic equipment and information equipment, there is a high demand primarily for small-size, light-weight, yet high energy-density secondary cells as the power supply for these equipment. In light of this demand, non-aqueous electrolyte secondary cells, especially lithium secondary cells, are drawing attention as a cell having a high voltage and high energy density.
As the positive active materials in the conventional lithium secondary cells, LiCoO.sub.2, LiNiO.sub.2, LiMn2O.sub.4, etc., are known. Cells employing LiCoO.sub.2 as the positive active material have already been commercialized. Also, since LiNiO.sub.2 as a positive active material provides lower cost and higher capacity than LiCoO.sub.2, it is an object of active research and development.
As an example, Japanese Laid-Open Patent No. Hei 8-138672 discloses a technology of manufacturing LiNiO.sub.2 as a positive active material in which a lithium hydroxide and a nickel salt are mixed in ethanol, dried, and pelletized, followed by temporal calcination at a temperature in the range 350 to 500.degree. C., and heating at a temperature in the range 750 to 850.degree. C.
Also, in U.S. Pat. No. 5,264,201 and in Japanese Laid-Open Patent No. Hei 6-342657, a method of synthesizing a lithium composite nickel-transition metal oxide is disclosed in which a mixture of a nickel oxide and nickel hydroxide or oxide or hydroxide of any one of Fe, Co, Cr, Ti, Mn, and V is used as the raw material, and the mixture and a lithium hydroxide are then mixed and heat treated at a temperature of 600.degree. C. or higher.
Also, in Japanese Laid-Open Patent No. Hei 8-185861, a technology of manufacturing LiNiO.sub.2 is disclosed in which a lithium compound and a nickel compound are mixed and calcinated at a temperature in the range 600 to 900.degree. C., and then calcinated at a temperature in the range 400 to 700.degree. C.
Furthermore, in Japanese Laid-Open Patent No. Hei 8-153513, a method of synthesizing a positive active material is disclosed in which a lithium compound and a metal salt, metal oxide or metal hydroxide are mixed and heat treated at a temperature in the range 600 to 1100.degree. C.
As the conventional process of manufacturing positive active materials for non-aqueous electrolyte secondary cells is based on a technology of synthesizing a lithium composite metal oxide using a lithium compound, when the particle size of the lithium compound used as the main material is large, for instance, lithium compound particles with a D50 value greater than 50 .mu.m, or a D90 value greater than 90 .mu.m, or with a particle size 100 .mu.m or greater exist, the contact area between the lithium compound and the mixed metal carbonates, metal oxides, or metal hydroxides becomes small. As a result, it suffered a problem of segregation of a lithium salt within the obtained positive active material, causing insufficient synthesizing reaction and lowering the capacity per unit weight of the active material. Here, D50 and D90 values represent particle sizes at which cumulative volume of particles smaller than the designated size reach 50% and 90%, respectively.
Also, because of segregation of lithium within the positive active material, a lithium composite metal oxide is locally synthesized in which lithium content is smaller than the stoichiometric composition. The locally produced lithium composite metal oxide undergoes destruction of layer structure of crystals with repetition of charge and discharge, thus presenting a problem of obstructing diffusion of lithium ions and lowering charge-discharge cycle characteristic of a non-aqueous electrolyte secondary cell.
With a view to solving this problem, a method of synthesizing a positive active material has been proposed (for instance in Japanese Laid-Open Patent No. Hei 8-138672) in which the raw materials are made to come into full contact with each other by mixing a lithium compound and other raw materials in a solvent such as ethanol and making a slurry. However, as this process of manufacturing a positive active material mixes raw materials in a solvent it suffers a problem of requiring additional processes such as drying and pelletizing, thus increasing the number of synthesizing processes.